This talk introduces 3GPP "Local Call, Local Switch" and covers the capabilities of the Osmocom implementation.
Traditionally, in GSM the control plan and [voice] user plane share the same path. This means that the voice call user data between two subscribers within the same cell/area goes all the way into the core network (MSC) and from there all the way back to the same cell/area and the other subscriber.
3GPP designed LCLS as a remedy to that: The BSC can detect when two legs of the same call pass through it, and then (under control of the MSC) locally switch the voice call, saving significant capacity on the A interface back-haul.
Osmocom recently added 3GPP LCLS (Local Call, Local Switch) capabilities to OsmoBSC.